Spinning is a method of forming a sheet or tube of work into a particular shape by centering it on a forming die and pressing it against the die by a roller tool, while rotating it with the die, and has been widely used in manufacturing various kinds of parts and products from metal sheets. A spinning apparatus is an apparatus for forming a sheet or tube of work into a particular shape by centering it on a forming die and pressing it against the die by a roller tool, while rotating it with the die.
A spinning apparatus is usually composed of a spindle for rotating a forming die and a piece of work and a plurality of (usually two) mutually crossing direct-acting actuators for driving a roller tool to press the work against the die.
Hydraulic cylinders or ball screw mechanisms rotated by servo motors have hitherto been used as the direct-acting actuators for driving the roller tool in a spinning apparatus. The law of control employed for the direct-acting actuators has been of position or speed control.
The clearance between the forming die and the roller tool is a factor which is particularly difficult to select in view of the control of a spinning apparatus, as compared with the other spinning conditions. The clearance has to be controlled accurately in accordance with the wall thickness of the product to be manufactured, and seriously affects its quality. The allowable range for a proper clearance is very narrow particularly when a sheet having a small thickness has to be worked upon. It is, therefore, necessary to position the roller accurately relative to the forming die. It is also necessary to have accurate information on the cross-sectional shape of the forming die. Moreover, it is necessary to follow the desired route of the roller accurately when a working force is acting.
It is, however, difficult to predict the wall thickness distribution of any product accurately when the product has a complicated shape, or is made by a multi-cycle draw spinning method in which its material is deformed progressively. For these reasons, the experience of people working at the site of manufacture is considerably relied upon for the selection of the clearance and forming tests are often conducted for the adjustment of the clearance.
In order to overcome those problems and eliminate the necessity for the accurate positioning of a forming die and a roller tool relative to each other and realize a proper clearance between the die and the roller tool, even if no accurate information may be available on the wall thickness of the product as intended, I, the inventor of this invention, have already invented a spinning method characterized by controlling a force with which a roller tool is pressed against work (see Japanese Patent Office Official Gazette JP-A-2004-223558). The invention disclosed therein will hereinafter be called the “earlier-filed invention”.
As spinning is performed by rotating a forming die and work, a roller tool is usually held substantially in a fixed position radially of the die, while it is displaced over a certain distance when it is fed along the axis of rotation of the die. Therefore, it has hitherto been possible to make only axially symmetric products having a circular cross section having its center on the axis of rotation of a forming die when it is taken at right angles to that axis. If it is possible to make also products having other shapes, such as polygonal or oval, in their cross sections normal to the axis of rotation of the die, it will be possible to realize the versatility of the products which can be manufactured, and thereby expand the applicability of spinning.
As a solution, there has been proposed a method in which a roller tool is pressed against work with a fixed force by a hydraulic cylinder to follow the contour of a forming die to make a product having a cross section other than circular (see JP-B-4-22648). The invention disclosed therein will hereinafter be called the “publicly known invention”.
The earlier-filed and publicly known inventions share the feature of controlling the pressing force of the roller tool. However, in the known spinning apparatus in which hydraulic cylinders or ball screw mechanisms are used to drive the roller tool, it is difficult to constitute a force control of satisfactorily high responsiveness because of the sliding friction between the cylinders and pistons, fluid resistance in a piping system for a hydraulic fluid, responsiveness of servo valves, friction and backlash of ball screws, elasticity of the joints between motors and ball screws, etc.
In the case of making any product that is not circular in its cross section normal to the axis of rotation of a forming die, it can be made only at a lower speed, since it is necessary to lower than usual the rotating speed of the die and work to allow hydraulic cylinders or ball screws to expand or contract quickly in response to any variation in radial length of the die.